The onset and association of CMEs with sigmoidal active regions

Previous studies of active regions characterised by Soft X‐ray S or inverse‐S morphology [ Canfield et al., 1999], have found these regions to possess a higher probability of eruption. In such cases, CME launch has been inferred using X‐ray proxies to indicate eruption. Active regions observed during 1997, previously categorised as both sigmoidal and eruptive [ Canfield, 1999], have been selected for further study, incorporating SoHO‐LASCO, SoHO‐EIT and ground based H‐alpha data. Our results allow re‐classification into three main categories; sigmoidal, non‐sigmoidal and active regions appearing sigmoidal due to the projection of many loops. Although the reduced dataset size prevents a statistical measure of significance, we note that regions comprising a single S (or inverse‐S) shaped structure are more frequently associated with a CME than those classed as non‐sigmoidal. This motivates the study of a larger dataset and highlights the need for a quantitative observational definition of the term “sigmoid”.